Methylmercury is a known neurotoxicant that has been reported to both increase and increase apoptotic processes in lymphocytes and brain cells. In general, however, studies with this and other neurotoxicants have used primarily neurotoxic assessment and only rarely has the possibility that neurotoxicants could be immunotoxic as well been considered. The studies proposed would look at both endpoints associated with immunotoxicity and with neurotoxicity. These endpoints would include apoptosis in organs of the immune system (spleen, thymus, lymph nodes, and bone marrow) and the nervous system (brain) in toxicant exposed mice. Capability to reactivate chronic Toxoplasma gondii (ME49) infection and the subsequent development of active toxoplasmosis would also be examined. This infection causes cyst formation and lesions in the brain (which contributes to the dementia noted in some, but not all AIDS patients) and immunosuppression and neurotoxic environmental chemicals have both been suggested to have a role in this process. Dexamethasone would serve as the positive immunosuppressant for these experiments. Differences in responses (apoptosis in the brain and immune system), T. gondii cyst formation and brain lesions) among mice of different strains with different capabilities for apoptosis will be included among the investigation.